Scientists have so far detected at least 550 planets outside the solar system – and another 2,000-plus await confirmation. But how to pick out the ones that may be Earth-like havens for life? Here's what one team looks for in assessing any planet's potential habitability and its similarity to Earth's properties.

4.
What kind of chemistry takes place?

An environment that supports the formation of long chains of molecules, with carbon as a backbone, is preferred. In some cases, silicon might replace carbon. Carbon is particularly adept at bonding with nitrogen, sulfur, and phosphorous, forming a range of biologically important molecules. The abundance of these molecules and the basic elements that build them wafting through a planet's atmosphere would provide clues as to the planet's hospitality.

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